Enhanced contrast for object detection and characterization by optical imaging

ABSTRACT

Enhanced contrast between an object of interest and background surfaces visible in an image is provided using controlled lighting directed at the object. Exploiting the falloff of light intensity with distance, a light source (or multiple light sources), such as an infrared light source, can be positioned near one or more cameras to shine light onto the object while the camera(s) capture images. The captured images can be analyzed to distinguish object pixels from background pixels.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to, and the benefits of, U.S. Ser. No.61/724,068, filed on Nov. 8, 2012, the entire disclosure of which ishereby incorporated by reference. In addition, this application is acontinuation-in-part of U.S. patent application Ser. No. 13/414,485(filed on Mar. 7, 2012) and Ser. No. 13/724,357 (filed on Dec. 21,2012), and also claims priority to and the benefit of U.S. ProvisionalPatent Application Nos. 61/724,091 (filed on Nov. 8, 2012) and61/587,554 (filed on Jan. 17, 2012). The foregoing applications areincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present disclosure relates generally to imaging systems and inparticular to three-dimensional (3D) object detection, tracking andcharacterization using optical imaging.

BACKGROUND

Motion-capture systems are used in a variety of contexts to obtaininformation about the conformation and motion of various objects,including objects with articulating members, such as human hands orhuman bodies. Such systems generally include cameras to capturesequential images of an object in motion and computers to analyze theimages to create a reconstruction of an object's volume, position andmotion. For 3D motion capture, at least two cameras are typically used.

Image-based motion-capture systems rely on the ability to distinguish anobject of interest from a background. This is often achieved usingimage-analysis algorithms that detect edges, typically by comparingpixels to detect abrupt changes in color and/or brightness. Suchconventional systems, however, suffer performance degradation under manycommon circumstances, e.g., low contrast between the object of interestand the background and/or patterns in the background that may falselyregister as object edges.

In some instances, distinguishing object and background can befacilitated by “instrumenting” the object of interest, e.g., by having aperson wear a mesh of reflectors or active light sources or the likewhile performing the motion. Special lighting conditions (e.g., lowlight) can be used to make the reflectors or light sources stand out inthe images. Instrumenting the subject, however, is not always aconvenient or desirable option.

SUMMARY

Certain embodiments of the present invention relate to imaging systemsthat improve object recognition by enhancing contrast between the objectand background surfaces visible in an image using; this may beaccomplished, for example, by means of controlled lighting directed atthe object. For example, in a motion-capture system where an object ofinterest, such as a person's hand, is significantly closer to the camerathan any background surfaces, the falloff of light intensity withdistance (1/r² for pointlike light sources) can be exploited bypositioning a light source (or multiple light sources) near thecamera(s) or other image capture device(s) and shining that light ontothe object. Source light reflected by the nearby object of interest canbe expected to be much brighter than light reflected from more distantbackground surfaces, and the more distant the background (relative tothe object), the more pronounced the effect will be. Accordingly, insome embodiments, a threshold cutoff on pixel brightness in the capturedimages can be used to distinguish “object” pixels from “background”pixels. While broadband ambient light sources can be employed, variousembodiments employ light having a confined wavelength range and a cameramatched to detect such light; for example, an infrared source light canbe used with one or more cameras sensitive to infrared frequencies.

Accordingly, in a first aspect, the invention pertains to an imagecapture and analysis system for identifying objects of interest in adigitally represented image scene. In various embodiments, the systemcomprises at least one camera oriented toward a field of view; at leastone light source disposed on a same side of the field of view as thecamera and oriented to illuminate the field of view; and an imageanalyzer coupled to the camera and the light source(s). The imageanalyzer may be configured to operate the camera(s) to capture asequence of images including a first image captured at a time when thelight source(s) are illuminating the field of view; identify pixelscorresponding to the object rather than to the background (e.g., imagecomponents that are nearby or reflective); and based on the identifiedpixels, constructing a 3D model of the object, including a position andshape of the object, to geometrically determine whether it correspondsto the object of interest. In certain embodiments, the image analyzerdistinguishes between (i) foreground image components corresponding toobjects located within a proximal zone of the field of view, where theproximal zone extends from the camera(s) and has a depth relativethereto of at least twice the expected maximum distance between theobjects corresponding to the foreground image components and thecamera(s), and (ii) background image components corresponding to objectslocated within a distal zone of the field of view, where the distal zoneis located, relative to the at least one camera, beyond the proximalzone. For example, the proximal zone may have a depth of at least fourtimes the expected maximum distance.

In other embodiments, the image analyzer operates the camera(s) tocapture second and third images when the light source(s) are notilluminating the field of view and identifies the pixels correspondingto the object based on the difference between the first and secondimages and the difference between the first and third images, where thesecond image is captured before the first image and the third image iscaptured after the second image.

The light source(s) may, for example, be diffuse emitters—e.g., infraredlight-emitting diodes, in which case the camera(s) are aninfrared-sensitive camera. Two or more light sources may be arranged toflank the camera(s) and be substantially coplanar therewith. In variousembodiments, the camera(s) and the light source(s) are orientedvertically upward. To enhance contrast, the camera may be operated toprovide an exposure time no greater than 100 microseconds and the lightsource(s) may be activated during exposure time at a power level of atleast 5 watts. In certain implementations, a holographic diffractiongrating is positioned between the lens of each camera and the field ofview (i.e., in front of the camera lens).

The image analyzer may geometrically determine whether an objectcorresponds to the object of interest by identifying ellipses thatvolumetrically define a candidate object, discarding object segmentsgeometrically inconsistent with an ellipse-based definition, anddetermining, based on the ellipses, whether the candidate objectcorresponds to the object of interest.

In another aspect, the invention pertains to a method for capturing andanalyzing images. In various embodiments, the method comprises the stepsof activating at least one light source to illuminate a field of viewcontaining an object of interest; capturing a sequence of digital imagesof the field of view using a camera (or cameras) at a time when thelight source(s) are activated; identifying pixels corresponding to theobject rather than to the background; and based on the identifiedpixels, constructing a 3D model of the object, including a position andshape of the object, to geometrically determine whether it correspondsto the object of interest.

The light source(s) may be positioned such that objects of interest arelocated within a proximal zone of the field of view, where the proximalzone extends from the camera to a distance at least twice an expectedmaximum distance between the objects of interest and the camera. Forexample, the proximal zone may have a depth of at least four times theexpected maximum distance. The light source(s) may, for example, bediffuse emitters—e.g., infrared light-emitting diodes, in which case thecamera is an infrared-sensitive camera. Two or more light sources may bearranged to flank the camera and be substantially coplanar therewith. Invarious embodiments, the camera and the light source(s) are orientedvertically upward. To enhance contrast, the camera may be operated toprovide an exposure time no greater than 100 microseconds and the lightsource(s) may be activated during exposure time at a power level of atleast 5 watts.

Alternatively, object pixels may be identified by capturing a firstimage when the light source(s) are not activated, a second image whenthe light source(s) are activated, and a third image when the lightsource(s) are not activated, where pixels corresponding to the objectare identified based on a difference between the second and first imagesand a difference between the second and third images

Geometrically determining whether an object corresponds to the object ofinterest may comprise or consist of identifying ellipses thatvolumetrically define a candidate object, discarding object segmentsgeometrically inconsistent with an ellipse-based definition, anddetermining, based on the ellipses, whether the candidate objectcorresponds to the object of interest.

In still another aspect the invention pertains to a method of locatingrounded objects within a digital image. In various embodiments, themethod comprises the steps of: activating at least one light source toilluminate a field of view containing an object of interest; operating acamera to capture a sequence of images including a first image capturedat a time when the at least one light source is illuminating the fieldof view; and analyzing the images to detect therein Gaussian brightnessfalloff patterns indicative of rounded objects in the field of view. Insome embodiments, the rounded objects are detected without identifyingedges thereof. The method may further comprise tracking the motion ofthe detected rounded objects through a plurality of the captured images.

Another aspect of the invention relates to an image capture and analysissystem for locating rounded objects within a field of view. In variousembodiments, the system comprises at least one camera oriented towardthe field of view; at least one light source disposed on a same side ofthe field of view as the camera and oriented to illuminate the field ofview; and an image analyzer coupled to the camera and the light source.The image analyzer may be configured to operate the camera(s) to capturea sequence of images including a first image captured at a time when thelight source(s) are illuminating the field of view; and analyze theimages to detect therein Gaussian brightness falloff patterns indicativeof rounded objects in the field of view. The rounded objects may, insome embodiments, be detected without identifying edges thereof. Thesystem may track the motion of the detected rounded objects through aplurality of the captured images.

As used herein, the term “substantially” or “approximately” means±10%(e.g., by weight or by volume), and in some embodiments, ±5%. The term“consists essentially of” means excluding other materials thatcontribute to function, unless otherwise defined herein. Referencethroughout this specification to “one example,” “an example,” “oneembodiment,” or “an embodiment” means that a particular feature,structure, or characteristic described in connection with the example isincluded in at least one example of the present technology. Thus, theoccurrences of the phrases “in one example,” “in an example,” “oneembodiment,” or “an embodiment” in various places throughout thisspecification are not necessarily all referring to the same example.Furthermore, the particular features, structures, routines, steps, orcharacteristics may be combined in any suitable manner in one or moreexamples of the technology. The headings provided herein are forconvenience only and are not intended to limit or interpret the scope ormeaning of the claimed technology.

The following detailed description together with the accompanyingdrawings will provide a better understanding of the nature andadvantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system for capturing image data according to anembodiment of the present invention.

FIG. 2 is a simplified block diagram of a computer system implementingan image analysis apparatus according to an embodiment of the presentinvention.

FIGS. 3A-3C are graphs of brightness data for rows of pixels that may beobtained according to an embodiment of the present invention.

FIG. 4 is a flow diagram of a process for identifying the location of anobject in an image according to an embodiment of the present invention.

FIG. 5 illustrates a timeline in which light sources pulsed on atregular intervals according to an embodiment of the present invention.

FIG. 6 illustrates a timeline for pulsing light sources and capturingimages according to an embodiment of the present invention.

FIG. 7 is a flow diagram of a process for identifying object edges usingsuccessive images according to an embodiment of the present invention.

FIG. 8 is a top view of a computer system incorporating a motiondetector as a user input device according to an embodiment of thepresent invention.

FIG. 9 is a front view of a tablet computer illustrating another exampleof a computer system incorporating a motion detector according to anembodiment of the present invention.

FIG. 10 illustrates a goggle system incorporating a motion detectoraccording to an embodiment of the present invention.

FIG. 11 is a flow diagram of a process for using motion information asuser input to control a computer system or other system according to anembodiment of the present invention.

FIG. 12 illustrates a system for capturing image data according toanother embodiment of the present invention.

FIG. 13 illustrates a system for capturing image data according to stillanother embodiment of the present invention.

DETAILED DESCRIPTION

Refer first to FIG. 1, which illustrates a system 100 for capturingimage data according to an embodiment of the present invention. System100 includes a pair of cameras 102, 104 coupled to an image-analysissystem 106. Cameras 102, 104 can be any type of camera, includingcameras sensitive across the visible spectrum or, more typically, withenhanced sensitivity to a confined wavelength band (e.g., the infrared(IR) or ultraviolet bands); more generally, the term “camera” hereinrefers to any device (or combination of devices) capable of capturing animage of an object and representing that image in the form of digitaldata. For example, line sensors or line cameras rather than conventionaldevices that capture a two-dimensional (2D) image can be employed. Theterm “light” is used generally to connote any electromagnetic radiation,which may or may not be within the visible spectrum, and may bebroadband (e.g., white light) or narrowband (e.g., a single wavelengthor narrow band of wavelengths).

The heart of a digital camera is an image sensor, which contains a gridof light-sensitive picture elements (pixels). A lens focuses light ontothe surface of the image sensor, and the image is formed as the lightstrikes the pixels with varying intensity. Each pixel converts the lightinto an electric charge whose magnitude reflects the intensity of thedetected light, and collects that charge so it can be measured. Both CCDand CMOS image sensors perform this same function but differ in how thesignal is measured and transferred.

In a CCD, the charge from each pixel is transported to a singlestructure that converts the charge into a measurable voltage. This isdone by sequentially shifting the charge in each pixel to its neighbor,row by row and then column by column in “bucket brigade” fashion, untilit reaches the measurement structure. A CMOS sensor, by contrast, placesa measurement structure at each pixel location. The measurements aretransferred directly from each location to the output of the sensor.

Cameras 102, 104 are preferably capable of capturing video images (i.e.,successive image frames at a constant rate of at least 15 frames persecond), although no particular frame rate is required. The capabilitiesof cameras 102, 104 are not critical to the invention, and the camerascan vary as to frame rate, image resolution (e.g., pixels per image),color or intensity resolution (e.g., number of bits of intensity dataper pixel), focal length of lenses, depth of field, etc. In general, fora particular application, any cameras capable of focusing on objectswithin a spatial volume of interest can be used. For instance, tocapture motion of the hand of an otherwise stationary person, the volumeof interest might be defined as a cube approximately one meter on aside.

System 100 also includes a pair of light sources 108, 110, which can bedisposed to either side of cameras 102, 104, and controlled byimage-analysis system 106. Light sources 108, 110 can be infrared lightsources of generally conventional design, e.g., infrared light-emittingdiodes (LEDs), and cameras 102, 104 can be sensitive to infrared light.Filters 120, 122 can be placed in front of cameras 102, 104 to filterout visible light so that only infrared light is registered in theimages captured by cameras 102, 104. In some embodiments where theobject of interest is a person's hand or body, use of infrared light canallow the motion-capture system to operate under a broad range oflighting conditions and can avoid various inconveniences or distractionsthat may be associated with directing visible light into the regionwhere the person is moving. However, a particular wavelength or regionof the electromagnetic spectrum is required.

It should be stressed that the foregoing arrangement is representativeand not limiting. For example, lasers or other light sources can be usedinstead of LEDs. For laser setups, additional optics (e.g., a lens ordiffuser) may be employed to widen the laser beam (and make its field ofview similar to that of the cameras). Useful arrangements can alsoinclude short- and wide-angle illuminators for different ranges. Lightsources are typically diffuse rather than specular point sources; forexample, packaged LEDs with light-spreading encapsulation are suitable.

In operation, cameras 102, 104 are oriented toward a region of interest112 in which an object of interest 114 (in this example, a hand) and oneor more background objects 116 can be present. Light sources 108, 110are arranged to illuminate region 112. In some embodiments, one or moreof the light sources 108, 110 and one or more of the cameras 102, 104are disposed below the motion to be detected, e.g., where hand motion isto be detected, beneath the spatial region where that motion takesplace. This is an optimal location because the amount of informationrecorded about the hand is proportional to the number of pixels itoccupies in the camera images, the hand will occupy more pixels when thecamera's angle with respect to the hand's “pointing direction” is asclose to perpendicular as possible. Because it is uncomfortable for auser to orient his palm toward a screen, the optimal positions areeither from the bottom looking up, from the top looking down (whichrequires a bridge) or from the screen bezel looking diagonally up ordiagonally down. In scenarios looking up there is less likelyhood ofconfusion with background objects (clutter on the user's desk, forexample) and if it is directly looking up then there is littlelikelihood of confusion with other people out of the field of view (andalso privacy is enhanced by not imaging faces). Image-analysis system106, which can be, e.g., a computer system, can control the operation oflight sources 108, 110 and cameras 102, 104 to capture images of region112. Based on the captured images, image-analysis system 106 determinesthe position and/or motion of object 114.

For example, as a step in determining the position of object 114,image-analysis system 106 can determine which pixels of various imagescaptured by cameras 102, 104 contain portions of object 114. In someembodiments, any pixel in an image can be classified as an “object”pixel or a “background” pixel depending on whether that pixel contains aportion of object 114 or not. With the use of light sources 108, 110,classification of pixels as object or background pixels can be based onthe brightness of the pixel. For example, the distance (r_(O)) betweenan object of interest 114 and cameras 102, 104 is expected to be smallerthan the distance (r_(B)) between background object(s) 116 and cameras102, 104. Because the intensity of light from sources 108, 110 decreasesas 1/r², object 114 will be more brightly lit than background 116, andpixels containing portions of object 114 (i.e., object pixels) will becorrespondingly brighter than pixels containing portions of background116 (i.e., background pixels). For example, if r_(B)/r_(O)=2, thenobject pixels will be approximately four times brighter than backgroundpixels, assuming object 114 and background 116 are similarly reflectiveof the light from sources 108, 110, and further assuming that theoverall illumination of region 112 (at least within the frequency bandcaptured by cameras 102, 104) is dominated by light sources 108, 110.These assumptions generally hold for suitable choices of cameras 102,104, light sources 108, 110, filters 120, 122, and objects commonlyencountered. For example, light sources 108, 110 can be infrared LEDscapable of strongly emitting radiation in a narrow frequency band, andfilters 120, 122 can be matched to the frequency band of light sources108, 110. Thus, although a human hand or body, or a heat source or otherobject in the background, may emit some infrared radiation, the responseof cameras 102, 104 can still be dominated by light originating fromsources 108, 110 and reflected by object 114 and/or background 116.

In this arrangement, image-analysis system 106 can quickly andaccurately distinguish object pixels from background pixels by applyinga brightness threshold to each pixel. For example, pixel brightness in aCMOS sensor or similar device can be measured on a scale from 0.0 (dark)to 1.0 (fully saturated), with some number of gradations in betweendepending on the sensor design. The brightness encoded by the camerapixels scales standardly (linearly) with the luminance of the object,typically due to the deposited charge or diode voltages. In someembodiments, light sources 108, 110 are bright enough that reflectedlight from an object at distance r_(O) produces a brightness level of1.0 while an object at distance r_(B)=2r_(O) produces a brightness levelof 0.25. Object pixels can thus be readily distinguished from backgroundpixels based on brightness. Further, edges of the object can also bereadily detected based on differences in brightness between adjacentpixels, allowing the position of the object within each image to bedetermined. Correlating object positions between images from cameras102, 104 allows image-analysis system 106 to determine the location in3D space of object 114, and analyzing sequences of images allowsimage-analysis system 106 to reconstruct 3D motion of object 114 usingconventional motion algorithms.

It will be appreciated that system 100 is illustrative and thatvariations and modifications are possible. For example, light sources108, 110 are shown as being disposed to either side of cameras 102, 104.This can facilitate illuminating the edges of object 114 as seen fromthe perspectives of both cameras; however, a particular arrangement ofcameras and lights is not required. (Examples of other arrangements aredescribed below.) As long as the object is significantly closer to thecameras than the background, enhanced contrast as described herein canbe achieved.

Image-analysis system 106 (also referred to as an image analyzer) caninclude or consist of any device or device component that is capable ofcapturing and processing image data, e.g., using techniques describedherein. FIG. 2 is a simplified block diagram of a computer system 200,implementing image-analysis system 106 according to an embodiment of thepresent invention. Computer system 200 includes a processor 202, amemory 204, a camera interface 206, a display 208, speakers 209, akeyboard 210, and a mouse 211.

Memory 204 can be used to store instructions to be executed by processor202 as well as input and/or output data associated with execution of theinstructions. In particular, memory 204 contains instructions,conceptually illustrated as a group of modules described in greaterdetail below, that control the operation of processor 202 and itsinteraction with the other hardware components. An operating systemdirects the execution of low-level, basic system functions such asmemory allocation, file management and operation of mass storagedevices. The operating system may be or include a variety of operatingsystems such as Microsoft WINDOWS operating system, the Unix operatingsystem, the Linux operating system, the Xenix operating system, the IBMAIX operating system, the Hewlett Packard UX operating system, theNovell NETWARE operating system, the Sun Microsystems SOLARIS operatingsystem, the OS/2 operating system, the BeOS operating system, theMACINTOSH operating system, the APACHE operating system, an OPENSTEPoperating system or another operating system of platform.

The computing environment may also include other removable/nonremovable,volatile/nonvolatile computer storage media. For example, a hard diskdrive may read or write to nonremovable, nonvolatile magnetic media. Amagnetic disk drive may read from or writes to a removable, nonvolatilemagnetic disk, and an optical disk drive may read from or write to aremovable, nonvolatile optical disk such as a CD-ROM or other opticalmedia. Other removable/nonremovable, volatile/nonvolatile computerstorage media that can be used in the exemplary operating environmentinclude, but are not limited to, magnetic tape cassettes, flash memorycards, digital versatile disks, digital video tape, solid state RAM,solid state ROM, and the like. The storage media are typically connectedto the system bus through a removable or non-removable memory interface.

Processor 202 may be a general-purpose microprocessor, but depending onimplementation can alternatively be a microcontroller, peripheralintegrated circuit element, a CSIC (customer-specific integratedcircuit), an ASIC (application-specific integrated circuit), a logiccircuit, a digital signal processor, a programmable logic device such asan FPGA (field-programmable gate array), a PLD (programmable logicdevice), a PLA (programmable logic array), an RFID processor, smartchip, or any other device or arrangement of devices that is capable ofimplementing the steps of the processes of the invention.

Camera interface 206 can include hardware and/or software that enablescommunication between computer system 200 and cameras such as cameras102, 104 shown in FIG. 1, as well as associated light sources such aslight sources 108, 110 of FIG. 1. Thus, for example, camera interface206 can include one or more data ports 216, 218 to which cameras can beconnected, as well as hardware and/or software signal processors tomodify data signals received from the cameras (e.g., to reduce noise orreformat data) prior to providing the signals as inputs to aconventional motion-capture (“mocap”) program 214 executing on processor202. In some embodiments, camera interface 206 can also transmit signalsto the cameras, e.g., to activate or deactivate the cameras, to controlcamera settings (frame rate, image quality, sensitivity, etc.), or thelike. Such signals can be transmitted, e.g., in response to controlsignals from processor 202, which may in turn be generated in responseto user input or other detected events.

Camera interface 206 can also include controllers 217, 219, to whichlight sources (e.g., light sources 108, 110) can be connected. In someembodiments, controllers 217, 219 supply operating current to the lightsources, e.g., in response to instructions from processor 202 executingmocap program 214. In other embodiments, the light sources can drawoperating current from an external power supply (not shown), andcontrollers 217, 219 can generate control signals for the light sources,e.g., instructing the light sources to be turned on or off or changingthe brightness. In some embodiments, a single controller can be used tocontrol multiple light sources.

Instructions defining mocap program 214 are stored in memory 204, andthese instructions, when executed, perform motion-capture analysis onimages supplied from cameras connected to camera interface 206. In oneembodiment, mocap program 214 includes various modules, such as anobject detection module 222 and an object analysis module 224; again,both of these modules are conventional and well-characterized in theart. Object detection module 222 can analyze images (e.g., imagescaptured via camera interface 206) to detect edges of an object thereinand/or other information about the object's location. Object analysismodule 224 can analyze the object information provided by objectdetection module 222 to determine the 3D position and/or motion of theobject. Examples of operations that can be implemented in code modulesof mocap program 214 are described below. Memory 204 can also includeother information and/or code modules used by mocap program 214.

Display 208, speakers 209, keyboard 210, and mouse 211 can be used tofacilitate user interaction with computer system 200. These componentscan be of generally conventional design or modified as desired toprovide any type of user interaction. In some embodiments, results ofmotion capture using camera interface 206 and mocap program 214 can beinterpreted as user input. For example, a user can perform hand gesturesthat are analyzed using mocap program 214, and the results of thisanalysis can be interpreted as an instruction to some other programexecuting on processor 200 (e.g., a web browser, word processor, orother application). Thus, by way of illustration, a user might useupward or downward swiping gestures to “scroll” a webpage currentlydisplayed on display 208, to use rotating gestures to increase ordecrease the volume of audio output from speakers 209, and so on.

It will be appreciated that computer system 200 is illustrative and thatvariations and modifications are possible. Computer systems can beimplemented in a variety of form factors, including server systems,desktop systems, laptop systems, tablets, smart phones or personaldigital assistants, and so on. A particular implementation may includeother functionality not described herein, e.g., wired and/or wirelessnetwork interfaces, media playing and/or recording capability, etc. Insome embodiments, one or more cameras may be built into the computerrather than being supplied as separate components. Further, an imageanalyzer can be implemented using only a subset of computer systemcomponents (e.g., as a processor executing program code, an ASIC, or afixed-function digital signal processor, with suitable I/O interfaces toreceive image data and output analysis results).

While computer system 200 is described herein with reference toparticular blocks, it is to be understood that the blocks are definedfor convenience of description and are not intended to imply aparticular physical arrangement of component parts. Further, the blocksneed not correspond to physically distinct components. To the extentthat physically distinct components are used, connections betweencomponents (e.g., for data communication) can be wired and/or wirelessas desired.

Execution of object detection module 222 by processor 202 can causeprocessor 202 to operate camera interface 206 to capture images of anobject and to distinguish object pixels from background pixels byanalyzing the image data. FIGS. 3A-3C are three different graphs ofbrightness data for rows of pixels that may be obtained according tovarious embodiments of the present invention. While each graphillustrates one pixel row, it is to be understood that an imagetypically contains many rows of pixels, and a row can contain any numberof pixels; for instance, an HD video image can include 1080 rows having1920 pixels each.

FIG. 3A illustrates brightness data 300 for a row of pixels in which theobject has a single cross-section, such as a cross-section through apalm of a hand. Pixels in region 302, corresponding to the object, havehigh brightness while pixels in regions 304 and 306, corresponding tobackground, have considerably lower brightness. As can be seen, theobject's location is readily apparent, and the locations of the edges ofthe object (at 308 at 310) are easily identified. For example, any pixelwith brightness above 0.5 can be assumed to be an object pixel, whileany pixel with brightness below 0.5 can be assumed to be a backgroundpixel.

FIG. 3B illustrates brightness data 320 for a row of pixels in which theobject has multiple distinct cross-sections, such as a cross-sectionthrough fingers of an open hand. Regions 322, 323, and 324,corresponding to the object, have high brightness while pixels inregions 326-329, corresponding to background, have low brightness.Again, a simple threshold cutoff on brightness (e.g., at 0.5) sufficesto distinguish object pixels from background pixels, and the edges ofthe object can be readily ascertained.

FIG. 3C illustrates brightness data 340 for a row of pixels in which thedistance to the object varies across the row, such as a cross-section ofa hand with two fingers extending toward the camera. Regions 342 and 343correspond to the extended fingers and have highest brightness; regions344 and 345 correspond to other portions of the hand and are slightlyless bright; this can be due in part to being farther away in part toshadows cast by the extended fingers. Regions 348 and 349 are backgroundregions and are considerably darker than hand-containing regions342-345. A threshold cutoff on brightness (e.g., at 0.5) again sufficesto distinguish object pixels from background pixels. Further analysis ofthe object pixels can also be performed to detect the edges of regions342 and 343, providing additional information about the object's shape.

It will be appreciated that the data shown in FIGS. 3A-3C isillustrative. In some embodiments, it may be desirable to adjust theintensity of light sources 108, 110 such that an object at an expecteddistance (e.g., r_(O) in FIG. 1) will be overexposed—that is, many ifnot all of the object pixels will be fully saturated to a brightnesslevel of 1.0. (The actual brightness of the object may in fact behigher.) While this may also make the background pixels somewhatbrighter, the 1/r² falloff of light intensity with distance still leadsto a ready distinction between object and background pixels as long asthe intensity is not set so high that background pixels also approachthe saturation level. As FIGS. 3A-3C illustrate, use of lightingdirected at the object to create strong contrast between object andbackground allows the use of simple and fast algorithms to distinguishbetween background pixels and object pixels, which can be particularlyuseful in real-time motion-capture systems. Simplifying the task ofdistinguishing background and object pixels can also free up computingresources for other motion-capture tasks (e.g., reconstructing theobject's position, shape, and/or motion).

Refer now to FIG. 4, which illustrates a process 400 for identifying thelocation of an object in an image according to an embodiment of thepresent invention. Process 400 can be implemented, e.g., in system 100of FIG. 1. At block 402, light sources 108, 110 are turned on. At block404, one or more images are captured using cameras 102, 104. In someembodiments, one image from each camera is captured. In otherembodiments, a sequence of images is captured from each camera. Theimages from the two cameras can be closely correlated in time (e.g.,simultaneous to within a few milliseconds) so that correlated imagesfrom the two cameras can be used to determine the 3D location of theobject.

At block 406, a threshold pixel brightness is applied to distinguishobject pixels from background pixels. Block 406 can also includeidentifying locations of edges of the object based on transition pointsbetween background and object pixels. In some embodiments, each pixel isfirst classified as either object or background based on whether itexceeds the threshold brightness cutoff. For example, as shown in FIGS.3A-3C, a cutoff at a saturation level of 0.5 can be used. Once thepixels are classified, edges can be detected by finding locations wherebackground pixels are adjacent to object pixels. In some embodiments, toavoid noise artifacts, the regions of background and object pixels oneither side of the edge may be required to have a certain minimum size(e.g., 2, 4 or 8 pixels).

In other embodiments, edges can be detected without first classifyingpixels as object or background. For example, Δβ can be defined as thedifference in brightness between adjacent pixels, and |Δβ| above athreshold (e.g., 0.3 or 0.5 in terms of the saturation scale) canindicate a transition from background to object or from object tobackground between adjacent pixels. (The sign of Δβ can indicate thedirection of the transition.) In some instances where the object's edgeis actually in the middle of a pixel, there may be a pixel with anintermediate value at the boundary. This can be detected, e.g., bycomputing two brightness values for a pixel i: βL=(βi+βi−1)/2 andβR=(βi+βi+1)/2, where pixel (i−1) is to the left of pixel i and pixel(i+1) is to the right of pixel i. If pixel i is not near an edge,|βL−βR| will generally be close to zero; if pixel is near an edge, then|βL−βR| will be closer to 1, and a threshold on |βL−βR| can be used todetect edges.

In some instances, one part of an object may partially occlude anotherin an image; for example, in the case of a hand, a finger may partlyocclude the palm or another finger. Occlusion edges that occur where onepart of the object partially occludes another can also be detected basedon smaller but distinct changes in brightness once background pixelshave been eliminated. FIG. 3C illustrates an example of such partialocclusion, and the locations of occlusion edges are apparent.

Detected edges can be used for numerous purposes. For example, aspreviously noted, the edges of the object as viewed by the two camerascan be used to determine an approximate location of the object in 3Dspace. The position of the object in a 2D plane transverse to theoptical axis of the camera can be determined from a single image, andthe offset (parallax) between the position of the object intime-correlated images from two different cameras can be used todetermine the distance to the object if the spacing between the camerasis known.

Further, the position and shape of the object can be determined based onthe locations of its edges in time-correlated images from two differentcameras, and motion (including articulation) of the object can bedetermined from analysis of successive pairs of images. Examples oftechniques that can be used to determine an object's position, shape andmotion based on locations of edges of the object are described inco-pending U.S. Ser. No. 13/414,485, filed Mar. 7, 2012, the entiredisclosure of which is incorporated herein by reference. Those skilledin the art with access to the present disclosure will recognize thatother techniques for determining position, shape and motion of an objectbased on information about the location of edges of the object can alsobe used.

In accordance with the '485 application, an object's motion and/orposition is reconstructed using small amounts of information. Forexample, an outline of an object's shape, or silhouette, as seen from aparticular vantage point can be used to define tangent lines to theobject from that vantage point in various planes, referred to herein as“slices.” Using as few as two different vantage points, four (or more)tangent lines from the vantage points to the object can be obtained in agiven slice. From these four (or more) tangent lines, it is possible todetermine the position of the object in the slice and to approximate itscross-section in the slice, e.g., using one or more ellipses or othersimple closed curves. As another example, locations of points on anobject's surface in a particular slice can be determined directly (e.g.,using a time-of-flight camera), and the position and shape of across-section of the object in the slice can be approximated by fittingan ellipse or other simple closed curve to the points. Positions andcross-sections determined for different slices can be correlated toconstruct a 3D model of the object, including its position and shape. Asuccession of images can be analyzed using the same technique to modelmotion of the object. Motion of a complex object that has multipleseparately articulating members (e.g., a human hand) can be modeledusing these techniques.

More particularly, an ellipse in the xy plane can be characterized byfive parameters: the x and y coordinates of the center (x_(C), y_(C)),the semimajor axis, the semiminor axis, and a rotation angle (e.g.,angle of the semimajor axis relative to the x axis). With only fourtangents, the ellipse is underdetermined. However, an efficient processfor estimating the ellipse in spite of this fact involves making aninitial working assumption (or “guess”) as to one of the parameters andrevisiting the assumption as additional information is gathered duringthe analysis. This additional information can include, for example,physical constraints based on properties of the cameras and/or theobject. In some circumstances, more than four tangents to an object maybe available for some or all of the slices, e.g., because more than twovantage points are available. An elliptical cross-section can still bedetermined, and the process in some instances is somewhat simplified asthere is no need to assume a parameter value. In some instances, theadditional tangents may create additional complexity. In somecircumstances, fewer than four tangents to an object may be availablefor some or all of the slices, e.g., because an edge of the object isout of range of the field of view of one camera or because an edge wasnot detected. A slice with three tangents can be analyzed. For example,using two parameters from an ellipse fit to an adjacent slice (e.g., aslice that had at least four tangents), the system of equations for theellipse and three tangents is sufficiently determined that it can besolved. As another option, a circle can be fit to the three tangents;defining a circle in a plane requires only three parameters (the centercoordinates and the radius), so three tangents suffice to fit a circle.Slices with fewer than three tangents can be discarded or combined withadjacent slices.

To determine geometrically whether an object corresponds to an object ofinterest comprises, one approach is to look for continuous volumes ofellipses that define an object and discard object segments geometricallyinconsistent with the ellipse-based definition of the object—e.g.,segments that are too cylindrical or too straight or too thin or toosmall or too far away—and discarding these. If a sufficient number ofellipses remain to characterize the object and it conforms to the objectof interest, it is so identified, and may be tracked from frame toframe.

In some embodiments, each of a number of slices is analyzed separatelyto determine the size and location of an elliptical cross-section of theobject in that slice. This provides an initial 3D model (specifically, astack of elliptical cross-sections), which can be refined by correlatingthe cross-sections across different slices. For example, it is expectedthat an object's surface will have continuity, and discontinuousellipses can accordingly be discounted. Further refinement can beobtained by correlating the 3D model with itself across time, e.g.,based on expectations related to continuity in motion and deformation.With renewed reference to FIGS. 1 and 2, in some embodiments, lightsources 108, 110 can be operated in a pulsed mode rather than beingcontinually on. This can be useful, e.g., if light sources 108, 110 havethe ability to produce brighter light in a pulse than in a steady-stateoperation. FIG. 5 illustrates a timeline in which light sources 108, 110are pulsed on at regular intervals as shown at 502. The shutters ofcameras 102, 104 can be opened to capture images at times coincidentwith the light pulses as shown at 504. Thus, an object of interest canbe brightly illuminated during the times when images are being captured.In some embodiments, the silhouettes of an object are extracted from oneor more images of the object that reveal information about the object asseen from different vantage points. While silhouettes can be obtainedusing a number of different techniques, in some embodiments, thesilhouettes are obtained by using cameras to capture images of theobject and analyzing the images to detect object edges.

In some embodiments, the pulsing of light sources 108, 110 can be usedto further enhance contrast between an object of interest andbackground. In particular, the ability to discriminate between relevantand irrelevant (e.g., background) objects in a scene can be compromisedif the scene contains object that themselves emit light or are highlyreflective. This problem can be addressed by setting the camera exposuretime to extraordinarily short periods (e.g., 100 microseconds or less)and pulsing the illumination at very high powers (i.e., 5 to 20 wattsor, in some cases, to higher levels, e.g., 40 watts). In this period oftime, most common sources of ambient illumination (e.g., fluorescentlights) are very dark by comparison to such bright, short-periodillumination; that is, in microseconds, non-pulsed light sources aredimmer than they would appear at an exposure time of milliseconds ormore. In effect, this approach increases the contrast of an object ofinterest with respect to other objects, even those emitting in the samegeneral band. Accordingly, discriminating by brightness under suchconditions allows irrelevant objects to be ignored for purposes of imagereconstruction and processing. Average power consumption is alsoreduced; in the case of 20 watts for 100 microseconds, the average powerconsumption is under 10 milliwatts. In general, the light sources 108,110 are operated so as to be on during the entire camera exposureperiod, i.e., the pulse width is equal to the exposure time and iscoordinated therewith.

It is also possible to coordinate pulsing of lights 108, 110 forpurposes of by comparing images taken with lights 108, 110 on and imagestaken with lights 108, 110 off.

FIG. 6 illustrates a timeline in which light sources 108, 110 are pulsedon at regular intervals as shown at 602, while shutters of cameras 102,104 are opened to capture images at times shown at 604. In this case,light sources 108, 110 are “on” for every other image. If the object ofinterest is significantly closer than background regions to lightsources 108, 110, the difference in light intensity will be stronger forobject pixels than for background pixels. Accordingly, comparing pixelsin successive images can help distinguish object and background pixels.

FIG. 7 is a flow diagram of a process 700 for identifying object edgesusing successive images according to an embodiment of the presentinvention. At block 702, the light sources are turned off, and at block704 a first image (A) is captured. Then, at block 706, the light sourcesare turned on, and at block 708 a second image (B) is captured. At block710, a “difference” image B−A is calculated, e.g., by subtracting thebrightness value of each pixel in image A from the brightness value ofthe corresponding pixel in image B. Since image B was captured withlights on, it is expected that B−A will be positive for most pixels.

The difference image is used to discriminate between background andforeground by applying a threshold or other metric on a pixel-by-pixelbasis. At block 712, a threshold is applied to the difference image(B−A) to identify object pixels, with (B−A) above a threshold beingassociated with object pixels and (B−A) below the threshold beingassociated with background pixels. Object edges can then be defined byidentifying where object pixels are adjacent to background pixels, asdescribed above. Object edges can be used for purposes such as positionand/or motion detection, as described above.

In an alternative embodiment, object edges are identified using atriplet of image frames rather than a pair. For example, in oneimplementation, a first image (Image1) is obtained with the lightsources turned off; a second image (Image2) is obtained with the lightsources turned on; and a third image (Image3) is taken with the lightsources again turned off. Two difference images,

Image4=abs(Image2−Image1) and

Image5=abs(Image2−Image3)

are then defined by subtracting pixel brightness values. A final image,Image6, is defined based on the two images Image4 and Image5. Inparticular, the value of each pixel in Image6 is the smaller of the twocorresponding pixel values in Image4 and Image5. In other words,Image6=min(Image4, Image5) on a pixel-by-pixel basis. Image6 representsan enhanced-accuracy difference image and most of its pixels will bepositive. Once again, a threshold or other metric can be used on apixel-by-pixel basis to distinguish foreground and background pixels.

Contrast-based object detection as described herein can be applied inany situation where objects of interest are expected to be significantlycloser (e.g., half the distance) to the light source(s) than backgroundobjects. One such application relates to the use of motion-detection asuser input to interact with a computer system. For example, the user maypoint to the screen or make other hand gestures, which can beinterpreted by the computer system as input.

A computer system 800 incorporating a motion detector as a user inputdevice according to an embodiment of the present invention isillustrated in FIG. 8. Computer system 800 includes a desktop box 802that can house various components of a computer system such asprocessors, memory, fixed or removable disk drives, video drivers, audiodrivers, network interface components, and so on. A display 804 isconnected to desktop box 802 and positioned to be viewable by a user. Akeyboard 806 is positioned within easy reach of the user's hands. Amotion-detector unit 808 is placed near keyboard 806 (e.g., behind, asshown or to one side), oriented toward a region in which it would benatural for the user to make gestures directed at display 804 (e.g., aregion in the air above the keyboard and in front of the monitor).Cameras 810, 812 (which can be similar or identical to cameras 102, 104described above) are arranged to point generally upward, and lightsources 814, 816 (which can be similar or identical to light sources108, 110 described above) are arranged to either side of cameras 810,812 to illuminate an area above motion-detector unit 808. In typicalimplementations, the cameras 810, 812 and the light sources 814, 816 aresubstantially coplanar. This configuration prevents the appearance ofshadows that can, for example, interfere with edge detection (as can bethe case were the light sources located between, rather than flanking,the cameras). A filter, not shown, can be placed over the top ofmotion-detector unit 808 (or just over the apertures of cameras 810,812) to filter out all light outside a band around the peak frequenciesof light sources 814, 816.

In the illustrated configuration, when the user moves a hand or otherobject (e.g., a pencil) in the field of view of cameras 810, 812, thebackground will likely consist of a ceiling and/or variousceiling-mounted fixtures. The user's hand can be 10-20 cm above motiondetector 808, while the ceiling may be five to ten times that distance(or more). Illumination from light sources 814, 816 will therefore bemuch more intense on the user's hand than on the ceiling, and thetechniques described herein can be used to reliably distinguish objectpixels from background pixels in images captured by cameras 810, 812. Ifinfrared light is used, the user will not be distracted or disturbed bythe light.

Computer system 800 can utilize the architecture shown in FIG. 1. Forexample, cameras 810, 812 of motion-detector unit 808 can provide imagedata to desktop box 802, and image analysis and subsequentinterpretation can be performed using the processors and othercomponents housed within desktop box 802. Alternatively, motion-detectorunit 808 can incorporate processors or other components to perform someor all stages of image analysis and interpretation. For example,motion-detector unit 808 can include a processor (programmable orfixed-function) that implements one or more of the processes describedabove to distinguish between object pixels and background pixels. Inthis case, motion-detector unit 808 can send a reduced representation ofthe captured images (e.g., a representation with all background pixelszeroed out) to desktop box 802 for further analysis and interpretation.A particular division of computational tasks between a processor insidemotion-detector unit 808 and a processor inside desktop box 802 is notrequired.

It is not always necessary to discriminate between object pixels andbackground pixels by absolute brightness levels; for example, whereknowledge of object shape exists, the pattern of brightness falloff canbe utilized to detect the object in an image even without explicitdetection of object edges. On rounded objects (such as hands andfingers), for example, the 1/r² relationship produces Gaussian ornear-Gaussian brightness distributions near the centers of the objects;imaging a cylinder illuminated by an LED and disposed perpendicularlywith respect to a camera results in an image having a bright center linecorresponding to the cylinder axis, with brightness falling off to eachside (around the cylinder circumference). Fingers are approximatelycylindrical, and by identifying these Gaussian peaks, it is possible tolocate fingers even in situations where the background is close and theedges are not visible due to the relative brightness of the background(either due to proximity or the fact that it may be actively emittinginfrared light). The term “Gaussian” is used broadly herein to connote acurve with a negative second derivative. Often such curves will bebell-shaped and symmetric, but this is not necessarily the case; forexample, in situations with higher object specularity or if the objectis at an extreme angle, the curve may be skewed in a particulardirection. Accordingly, as used herein, the term “Gaussian” is notlimited to curves explicitly conforming to a Gaussian function.

FIG. 9 illustrates a tablet computer 900 incorporating a motion detectoraccording to an embodiment of the present invention. Tablet computer 900has a housing, the front surface of which incorporates a display screen902 surrounded by a bezel 904. One or more control buttons 906 can beincorporated into bezel 904. Within the housing, e.g., behind displayscreen 902, tablet computer 900 can have various conventional computercomponents (processors, memory, network interfaces, etc.). A motiondetector 910 can be implemented using cameras 912, 914 (e.g., similar oridentical to cameras 102, 104 of FIG. 1) and light sources 916, 918(e.g., similar or identical to light sources 108, 110 of FIG. 1) mountedinto bezel 904 and oriented toward the front surface so as to capturemotion of a user positioned in front of tablet computer 900.

When the user moves a hand or other object in the field of view ofcameras 912, 914, the motion is detected as described above. In thiscase, the background is likely to be the user's own body, at a distanceof roughly 25-30 cm from tablet computer 900. The user may hold a handor other object at a short distance from display 902, e.g., 5-10 cm. Aslong as the user's hand is significantly closer than the user's body(e.g., half the distance) to light sources 916, 918, theillumination-based contrast enhancement techniques described herein canbe used to distinguish object pixels from background pixels. The imageanalysis and subsequent interpretation as input gestures can be donewithin tablet computer 900 (e.g., leveraging the main processor toexecute operating-system or other software to analyze data obtained fromcameras 912, 914). The user can thus interact with tablet 900 usinggestures in 3D space.

A goggle system 1000, as shown in FIG. 10, may also incorporate a motiondetector according to an embodiment of the present invention. Gogglesystem 1000 can be used, e.g., in connection with virtual-reality and/oraugmented-reality environments. Goggle system 1000 includes goggles 1002that are wearable by a user, similar to conventional eyeglasses. Goggles1002 include eyepieces 1004, 1006 that can incorporate small displayscreens to provide images to the user's left and right eyes, e.g.,images of a virtual reality environment. These images can be provided bya base unit 1008 (e.g., a computer system) that is in communication withgoggles 1002, either via a wired or wireless channel. Cameras 1010, 1012(e.g., similar or identical to cameras 102, 104 of FIG. 1) can bemounted in a frame section of goggles 1002 such that they do not obscurethe user's vision. Light sources 1014, 1016 can be mounted in the framesection of goggles 1002 to either side of cameras 1010, 1012. Imagescollected by cameras 1010, 1012 can be transmitted to base unit 1008 foranalysis and interpretation as gestures indicating user interaction withthe virtual or augmented environment. (In some embodiments, the virtualor augmented environment presented through eyepieces 1004, 1006 caninclude a representation of the user's hand, and that representation canbe based on the images collected by cameras 1010, 1012.)

When the user gestures using a hand or other object in the field of viewof cameras 1008, 1010, the motion is detected as described above. Inthis case, the background is likely to be a wall of a room the user isin, and the user will most likely be sitting or standing at somedistance from the wall. As long as the user's hand is significantlycloser than the user's body (e.g., half the distance) to light sources1012, 1014, the illumination-based contrast enhancement techniquesdescribed herein facilitate distinguishing object pixels from backgroundpixels. The image analysis and subsequent interpretation as inputgestures can be done within base unit 1008.

It will be appreciated that the motion-detector implementations shown inFIGS. 8-10 are illustrative and that variations and modifications arepossible. For example, a motion detector or components thereof can becombined in a single housing with other user input devices, such as akeyboard or trackpad. As another example, a motion detector can beincorporated into a laptop computer, e.g., with upward-oriented camerasand light sources built into the same surface as the laptop keyboard(e.g., to one side of the keyboard or in front of or behind it) or withfront-oriented cameras and light sources built into a bezel surroundingthe laptop's display screen. As still another example, a wearable motiondetector can be implemented, e.g., as a headband or headset that doesnot include active displays or optical components.

As illustrated in FIG. 11, motion information can be used as user inputto control a computer system or other system according to an embodimentof the present invention. Process 1100 can be implemented, e.g., incomputer systems such as those shown in FIGS. 8-10. At block 1102,images are captured using the light sources and cameras of the motiondetector. As described above, capturing the images can include using thelight sources to illuminate the field of view of the cameras such thatobjects closer to the light sources (and the cameras) are more brightlyilluminated than objects farther away.

At block 1104, the captured images are analyzed to detect edges of theobject based on changes in brightness. For example, as described above,this analysis can include comparing the brightness of each pixel to athreshold, detecting transitions in brightness from a low level to ahigh level across adjacent pixels, and/or comparing successive imagescaptured with and without illumination by the light sources. At block1106, an edge-based algorithm is used to determine the object's positionand/or motion. This algorithm can be, for example, any of thetangent-based algorithms described in the above-referenced '485application; other algorithms can also be used.

At block 1108, a gesture is identified based on the object's positionand/or motion. For example, a library of gestures can be defined basedon the position and/or motion of a user's fingers. A “tap” can bedefined based on a fast motion of an extended finger toward a displayscreen. A “trace” can be defined as motion of an extended finger in aplane roughly parallel to the display screen. An inward pinch can bedefined as two extended fingers moving closer together and an outwardpinch can be defined as two extended fingers moving farther apart. Swipegestures can be defined based on movement of the entire hand in aparticular direction (e.g., up, down, left, right) and different swipegestures can be further defined based on the number of extended fingers(e.g., one, two, all). Other gestures can also be defined. By comparinga detected motion to the library, a particular gesture associated withdetected position and/or motion can be determined.

At block 1110, the gesture is interpreted as user input, which thecomputer system can process. The particular processing generally dependson application programs currently executing on the computer system andhow those programs are configured to respond to particular inputs. Forexample, a tap in a browser program can be interpreted as selecting alink toward which the finger is pointing. A tap in a word-processingprogram can be interpreted as placing the cursor at a position where thefinger is pointing or as selecting a menu item or other graphicalcontrol element that may be visible on the screen. The particulargestures and interpretations can be determined at the level of operatingsystems and/or applications as desired, and no particular interpretationof any gesture is required.

Full-body motion can be captured and used for similar purposes. In suchembodiments, the analysis and reconstruction advantageously occurs inapproximately real-time (e.g., times comparable to human reactiontimes), so that the user experiences a natural interaction with theequipment. In other applications, motion capture can be used for digitalrendering that is not done in real time, e.g., for computer-animatedmovies or the like; in such cases, the analysis can take as long asdesired.

Embodiments described herein provide efficient discrimination betweenobject and background in captured images by exploiting the decrease oflight intensity with distance. By brightly illuminating the object usingone or more light sources that are significantly closer to the objectthan to the background (e.g., by a factor of two or more), the contrastbetween object and background can be increased. In some instances,filters can be used to remove light from sources other than the intendedsources. Using infrared light can reduce unwanted “noise” or brightspots from visible light sources likely to be present in the environmentwhere images are being captured and can also reduce distraction to users(who presumably cannot see infrared).

The embodiments described above provide two light sources, one disposedto either side of the cameras used to capture images of the object ofinterest. This arrangement can be particularly useful where the positionand motion analysis relies on knowledge of the object's edges as seenfrom each camera, as the light sources will illuminate those edges.However, other arrangements can also be used. For example, FIG. 12illustrates a system 1200 with a single camera 1202 and two lightsources 1204, 1206 disposed to either side of camera 1202. Thisarrangement can be used to capture images of object 1208 and shadowscast by object 1208 against a flat background region 1210. In thisembodiment, object pixels and background pixels can be readilydistinguished. In addition, provided that background 1210 is not too farfrom object 1208, there will still be enough contrast between pixels inthe shadowed background region and pixels in the unshadowed backgroundregion to allow discrimination between the two. Position and motiondetection algorithms using images of an object and its shadows aredescribed in the above-referenced '485 application and system 1200 canprovide input information to such algorithms, including the location ofedges of the object and its shadows.

The single-camera implementation 1200 may benefit from inclusion of aholographic diffraction grating 1215 placed in front of the lens of thecamera 1202. The grating 1215 creates fringe patterns that appear asghost silhouettes and/or tangents of the object 1208. Particularly whenseparable (i.e., when overlap is not excessive), these patterns providehigh contrast facilitating discrimination of object from background.See, e.g., DIFFRACTION GRATING HANDBOOK (Newport Corporation, January2005; available athttp://gratings.newport.com/library/handbook/handbook.asp), the entiredisclosure of which is hereby incorporated by reference.

FIG. 13 illustrates another system 1300 with two cameras 1302, 1304 andone light source 1306 disposed between the cameras. System 1300 cancapture images of an object 1308 against a background 1310. System 1300is generally less reliable for edge illumination than system 100 of FIG.1; however, not all algorithms for determining position and motion relyon precise knowledge of the edges of an object. Accordingly, system 1300can be used, e.g., with edge-based algorithms in situations where lessaccuracy is required. System 1300 can also be used with non-edge-basedalgorithms.

While the invention has been described with respect to specificembodiments, one skilled in the art will recognize that numerousmodifications are possible. The number and arrangement of cameras andlight sources can be varied. The cameras' capabilities, including framerate, spatial resolution, and intensity resolution, can also be variedas desired. The light sources can be operated in continuous or pulsedmode. The systems described herein provide images with enhanced contrastbetween object and background to facilitate distinguishing between thetwo, and this information can be used for numerous purposes, of whichposition and/or motion detection is just one among many possibilities.

Threshold cutoffs and other specific criteria for distinguishing objectfrom background can be adapted for particular cameras and particularenvironments. As noted above, contrast is expected to increase as theratio r_(B)/r_(O) increases. In some embodiments, the system can becalibrated in a particular environment, e.g., by adjusting light-sourcebrightness, threshold criteria, and so on. The use of simple criteriathat can be implemented in fast algorithms can free up processing powerin a given system for other uses.

Any type of object can be the subject of motion capture using thesetechniques, and various aspects of the implementation can be optimizedfor a particular object. For example, the type and positions of camerasand/or light sources can be optimized based on the size of the objectwhose motion is to be captured and/or the space in which motion is to becaptured. Analysis techniques in accordance with embodiments of thepresent invention can be implemented as algorithms in any suitablecomputer language and executed on programmable processors.Alternatively, some or all of the algorithms can be implemented infixed-function logic circuits, and such circuits can be designed andfabricated using conventional or other tools.

Computer programs incorporating various features of the presentinvention may be encoded on various computer readable storage media;suitable media include magnetic disk or tape, optical storage media suchas compact disk (CD) or DVD (digital versatile disk), flash memory, andany other non-transitory medium capable of holding data in acomputer-readable form. Computer-readable storage media encoded with theprogram code may be packaged with a compatible device or providedseparately from other devices. In addition program code may be encodedand transmitted via wired optical, and/or wireless networks conformingto a variety of protocols, including the Internet, thereby allowingdistribution, e.g., via Internet download.

Thus, although the invention has been described with respect to specificembodiments, it will be appreciated that the invention is intended tocover all modifications and equivalents within the scope of thefollowing claims.

What is claimed is:
 1. An image capture and analysis system foridentifying objects of interest in a digitally represented image scene,the system comprising: at least one camera oriented toward a field ofview; at least one light source disposed on a same side of the field ofview as the camera and oriented to illuminate the field of view; and animage analyzer coupled to the camera and the at least one light sourceand configured to: operate the at least one camera to capture a sequenceof images including a first image captured at a time when the at leastone light source is illuminating the field of view; identify pixelscorresponding to the object rather than to a background; and based onthe identified pixels, constructing a 3D model of the object, includinga position and shape of the object, to geometrically determine whetherit corresponds to the object of interest.
 2. The system of claim 1wherein the image analyzer distinguishes between (i) foreground imagecomponents corresponding to objects located within a proximal zone ofthe field of view, the proximal zone extending from the at least onecamera and having a depth relative thereto of at least twice an expectedmaximum distance between the objects corresponding to the foregroundimage components and the at least one camera, and (ii) background imagecomponents corresponding to objects located within a distal zone of thefield of view, the distal zone being located, relative to the at leastone camera, beyond the proximal zone.
 3. The system of claim 2 whereinthe proximal zone has a depth of at least four times the expectedmaximum distance.
 4. The system of claim 1 wherein the at least onelight source is a diffuse emitter.
 5. The system of claim 4 wherein theat least one light source is an infrared light-emitting diode and the atleast one camera is an infrared-sensitive camera.
 6. The system of claim1 wherein there are at least two light sources flanking the at least onecamera and substantially coplanar therewith.
 7. The system of claim 1wherein the at least one camera and the at least one light source areoriented vertically upward.
 8. The system of claim 1 wherein the atleast one camera is operated to provide an exposure time no greater than100 microseconds and the at least one light source is activated duringexposure time at a power level of at least 5 watts.
 9. The system ofclaim 1 further comprising a holographic diffraction grating positionedbetween a lens of the at least one camera and the field of view.
 10. Thesystem of claim 1 wherein the image analyzer operates the at least onecamera to capture second and third images when the at least one lightsource is not illuminating the field of view and identifies the pixelscorresponding to the object based on a difference between the first andsecond images and a difference between the first and third images,wherein the second image is captured before the first image and thethird image is captured after the second image.
 11. A method forcapturing and analyzing images, the method comprising the steps of:activating at least one light source to illuminate a field of viewcontaining an object of interest; capturing a sequence of digital imagesof the field of view using a camera at a time when the at least onelight source is activated; identifying pixels corresponding to theobject rather than to a background; and based on the identified pixels,constructing a 3D model of the object, including a position and shape ofthe object, to geometrically determine whether it corresponds to theobject of interest.
 12. The method of claim 11 wherein the at least onelight source is positioned such that objects of interest are locatedwithin a proximal zone of the field of view, the proximal zone extendingfrom a camera to a distance at least twice an expected maximum distancebetween the objects of interest and the camera.
 13. The method of claim12 wherein the proximal zone has a depth of at least four times theexpected maximum distance.
 14. The method of claim 11 wherein the atleast one light source is a diffuse emitter.
 15. The method of claim 11wherein the at least one light source is an infrared light-emittingdiode and the camera is an infrared-sensitive camera.
 16. The method ofclaim 11 wherein two light sources are activated, the light sourcesflanking the camera and substantially coplanar therewith.
 17. The methodof claim 11 wherein the camera and the at least one light source areoriented vertically upward.
 18. The method of claim 11 furthercomprising capturing a first image when the at least one light source isnot activated, a second image when the at least one light source isactivated, and a third image when the at least one light source is notactivated, wherein pixels corresponding to the object are identifiedbased on a difference between the second and first images and adifference between the second and third images.
 19. A method of locatingrounded objects within a digital image, the method comprising the stepsof: activating at least one light source to illuminate a field of viewcontaining an object of interest; operating a camera to capture asequence of images including a first image captured at a time when theat least one light source is illuminating the field of view; andanalyzing the images to detect therein Gaussian brightness falloffpatterns indicative of rounded objects in the field of view.
 20. Themethod of claim 19 wherein the rounded objects are detected withoutidentifying edges thereof.
 21. The method of claim 19 further comprisingtracking motion of the detected rounded objects through a plurality ofthe captured images.
 22. An image capture and analysis system forlocating rounded objects within a field of view, the system comprising:at least one camera oriented toward the field of view; at least onelight source disposed on a same side of the field of view as the cameraand oriented to illuminate the field of view; and an image analyzercoupled to the camera and the light source and configured to: operatethe at least one camera to capture a sequence of images including afirst image captured at a time when the at least one light source isilluminating the field of view; and analyze the images to detect thereinGaussian brightness falloff patterns indicative of rounded objects inthe field of view.
 23. The system of claim 22 wherein the roundedobjects are detected without identifying edges thereof.
 24. The systemof claim 22 further comprising tracking motion of the detected roundedobjects through a plurality of the captured images.